President Trump’s dismissive treatment of Europe is beginning to erode the transatlantic alliance, which for many decades has been the central pillar of U.S. national security policy.

The growing European-American rift may be the most important but least discussed consequence of Trump’s foreign policy. His disruptive style is usually seen as destabilizing to distant adversaries in Pyongyang, Tehran and Beijing. But the diplomatic bombs have also been exploding here in the capital of the European Union — as well as in Paris, Berlin and London — and they appear to be causing real damage.

Many European leaders have stopped being polite about Trump. After a year and a half of intermittent skirmishes, they’ve started firing back — describing Trump as a danger to Europe’s security interests and moving toward an open break with Washington on Iran and other key issues.

German Chancellor Angela Merkel said this month that Trump’s assault on the Iran deal had created a “real crisis” for the global order. French Finance Minister Bruno Le Maire said that unilateral U.S. imposition of sanctions as “economic policeman of the planet” is “not acceptable.”

European Council President Donald Tusk summed up the continental frustration and anger in a bitter tweet Wednesday: “Looking at latest decisions of [Trump] someone could even think: with friends like that who needs enemies. But frankly, [the European Union] should be grateful. Thanks to him we got rid of all illusions.”

The NATO military alliance still seems relatively solid. NATO Secretary General Jens Stoltenberg tweeted after visiting Trump on Thursday that it was a “good meeting” and credited the president’s “leadership on defense spending.” But how long can this defense amity continue if there’s an open break on major diplomatic issues?

The Europeans will take an important symbolic step away from the United States next week when representatives of Germany, France and Britain join Russia and China in a meeting of the joint commission that oversees the Iran nuclear agreement. “This is painful for us,” said a senior member of the European Union’s diplomatic service, in an interview here Wednesday.

European officials say that they don’t feel comfortable siding with Russia, China and Iran against the United States, but that the Trump administration’s withdrawal from the deal has given them no alternative. Europeans see the agreement as vital for their national security, because it checks the danger of nuclear breakout by Iran and other countries in a Middle East that’s all too close.

Europeans scrambled this week to reassure Iran and keep it in the deal partly because they don’t want to jeopardize its monitoring provision. Under the current inspection protocol, cameras record 2 million digital images a day at key sites in Iran, one official said. Thanks to the deal, this official estimated, Iran’s breakout time to build a bomb has stretched from a few weeks to a year. Europeans don’t want to risk losing that warning time by scuttling the agreement.

“The U.S. has never told us what Plan B is,” the E.U. diplomat said. “What do we do when [the Iranians] kick the inspectors out?”

The U.S.-European confrontation will deepen if Washington, as expected, imposes secondary sanctions against European companies that do business with Iran. If so, Europe might retaliate with “blocking regulations” that punish companies that comply with the American measures.

European companies don’t like being squeezed. Total, the giant French oil company that plans a multibillion-dollar natural gas project in Iran, said Wednesday that unless it gets “a specific project waiver” from the Treasury Department, it will have to scuttle its investment plan, which Total says has “huge potential.”

Trump is so unpopular in Europe that defying him carries little political risk. On issues such as trade, climate change, the Israeli-Palestinian conflict and global economic policy, the traditional centrist policy consensus has mostly held in Europe.

The transatlantic divide on culture and values, once the bedrock of the alliance, is striking. Trump, with his braggadocio and vulgarity, seems almost a caricature of a rough, violent United States that many Europeans dislike. A poll last year by the Pew Research Center found that only 11 percent of Germans, for example, trusted Trump to do the right thing, compared with 86 percent for his predecessor, Barack Obama.

Americans have taken European support for granted for so long that few analysts have examined what a real breach in the transatlantic alliance would look like. Maybe it’s time to consider the “what ifs.”

Trump has often said that “America First” doesn’t mean “America alone.” But Europe gets a vote on that, too, and this week it was resoundingly negative.